It has long been known to use carbon blacks as fillers in the compounding and preparation of elastomeric compositions, most notably for their properties as pigments and reenforcing agents. Carbon blacks are especially useful in the preparation of elastomeric compositions for the manufacture of motor vehicle tires and the like. Elastomeric compounds suitable for tire treads, for example, typically employ carbon black fillers as reenforcing agents to provide high abrasion resistance and good hysterisis balance at different temperatures. The physical properties of the carbon black directly influence the abrasion resistance and hysterisis of the tread compound. Generally, a carbon black with high surface area and small particle size will impart a high abrasion resistance and high hysterisis to the tread compound. Other applications where it is useful to provide an elastomer exhibiting good hysterisis balance include other tire components, such as under tread, wedge compounds, sidewall, carcass, apex, bead filler and wire skim, as well as engine mounts and base compounds used in industrial drive and automotive belts.
Reference here to the desirable property of good hysterisis balance, relates to the fact that elastomers are not completely elastic. Upon repeated deformation, only part of the energy is returned. The lost energy, hysterisis, usually manifests itself in the form of heat. This energy loss plays a positive roll in providing good tire traction, but unfortunately, also occurs during rolling of the tire, resulting in undesirable rolling resistance. Thus, the hysterisis of an elastomeric compound under cyclic deformation is the difference between the energy applied to deform the elastomeric composition and the energy released as the elastomeric composition recovers to its initial undeformed state. Hysterisis is known to be well-characterized by a loss tangent, tan .delta., the ratio of the loss modulus to the storage modulus, that is, viscous modulus to elastic modulus. Also characterized as the ratio of energy lost (G") to energy returned (G'), the loss factor tan .delta. is widely used to indicate tire performance properties. Tan .delta. values at low temperatures (for example, -30.degree. C. to 0.degree. C.) are used as an indication of wet traction capability, with higher values being desirable. For rolling resistance, typically, measurement may be based on a temperature from the range of 30.degree. C. to 70.degree. C. However, the amplitude of deformation also has a significant effect on hysterisis, so it is known to test a strain sweep from low to high at one or more fixed temperatures. The highest value, tan .delta. max, is an indicator of rolling resistance, with low tan .delta. max values being desirable as corresponding to low rolling resistance.
Tires made with a tire tread compound having lower hysterisis measured at higher temperatures, such as 40.degree. C. or higher, will have correspondingly lower rolling resistance, which in turn results in reduced fuel consumption by a vehicle using the tire. A tire tread with higher hysterisis value measured at low temperature, such as 0.degree. C. or lower, will have better wet traction and skid resistance. A tire tread compound demonstrating both low hysterisis at high temperatures and high hysterisis at low temperatures is said to have good hysterisis balance. Since it is highly desirable to provide reenforced elastomeric compositions suitable for tire tread applications and the like, having simultaneously both excellent traction and low rolling resistance, compositions are sought which exhibit high hysterisis at traction conditions and low hysterisis at higher speed rolling conditions. Stated in terms of hysterisis balance, it has long been an objective to develop elastomeric compositions, especially for tire tread application or the like, having higher tan .delta. values at low temperature, low frequency conditions for good traction, along with lower tan .delta. max values as measured, for example, at 60.degree. C., with low lost energy (G") versus returned energy (G') for good (that is, low) rolling resistance.
Various grades of silica also are known and used as fillers for elastomeric compositions. Silica alone as a reenforcing agent for elastomer typically yields compositions having poor performance characteristics for tire applications, compared to the results obtained with carbon black alone as a reenforcing agent. It has been theorized that strong filler-filler interaction and poor filler-elastomer interaction may account for the poor performance of silica alone. The silica-elastomer interaction can be improved by chemically bonding the two with a silane coupling agent, such as bis(3-triethoxysilylpropyl) tetra-sulfane, commercially available as Si-69 from Degussa AG, Germany. Coupling agents such as Si-69 are understood to create a chemical linkage between the elastomer and the silica, thereby coupling the silica to the elastomer. When the silica is chemically coupled to the elastomer, certain performance characteristics of the resulting elastomeric composition are enhanced. When incorporated into vehicle tires, for example, such elastomeric compounds provide improved hysterisis balance. Unfortunately, silica fillers typically are more expensive than comparable carbon black fillers, resulting often in an undesirable cost penalty for their use in elastomeric compositions. In addition, silane coupling agents such as Si-69 are quite costly, further exacerbating the cost penalty. Also, elastomer compounds containing silica as the primary reenforcing agent have been found generally to exhibit low thermal conductivity, high electrical resistivity, high density and poor process ability.
Coupling agents suitable for silica fillers are discussed, for example, in U.S. Pat. No. 5,328,949 to Sandstrom et al. As noted there, such coupling agents are generally composed of a silane compound having a constituent component or moiety (the silane portion) capable of reacting with the silica surface and, also, a constituent component or moiety capable of reacting with the elastomer molecule, particularly a sulfur vulcanizable rubber having carbon-to-carbon double bonds or unsaturation. In this manner, the Sandstrom patent states, the coupling agent acts as a connecting bridge between the silica and the rubber and thereby enhances the rubber reinforcement performance of the silica filler. Dithiodipropionic acid is disclosed as the silica coupling agent. The Sandstrom et al. patent further notes the optional inclusion of traditional silane coupling agents along with the dithiodipropionic acid, and carbon black filler along with the silica filler. A report by the Malaysian Rubber Producers Research Association ("the MRPRA report"), Functionalization of Elastomers by Reactive Mixing, Research Disclosure--June 1994 (p. 308) shows a vulcanized 60:40 natural rubber:EPDM elastomer blend comprising 50 phr N660 carbon black filler to have less bound rubber (g/g black) in the natural rubber portion and more in the EPDM portion when modified by reaction with chemicals currently employed in accelerated sulfur vulcanization of rubber compounds, including bis-4-(1,1-dimethyl(propyl)phenoldisulfide ("BAPD") and dithiodimorpholine ("DTDM"). The use of dithiodicaprolactam ("DTDC") is shown to yield an increase in both. The modification by mixing at temperatures in excess of 150.degree. C. is said to yield improved properties in the ultimate vulcanizates. An increase is reported for both SPR and EPDM-1 through modification of the elastomer with dithiodicaprolactam during mixing of the elastomer with 50 phr N330 carbon black. Other additives have been suggested for use together with curatives or a vulcanization system, including BCI-MX sold under the trade name Perkalink 900, Akzo Nobel Chemicals, Inc., Akron, Ohio, USA. Such BCI-MX additive is said to serve as an antireversion agent during curing of a composition incorporating CBS, 6PPD, APDS, carbon black (N-375), aromatic oil (Dutrex 729 HP), zinc oxide, stearic acid and sulfur. The MRPRA report and another such report were characterized in Rubber Reviews (published by the Rubber Division, American Chemical Society) as showing modification of elastomers with sulfur donors by mixing at the elevated temperatures typical of the preparation of masterbatches in an internal mixer to achieve low levels of modification both in the absence and presence of carbon black during mixing. Such modification of the elastomers is analogized there to elastomer modification wherein a function (morpholine, caprolactam or alkylphenol mono-sulfide) is bound to the rubber via a sulfur link, and this function later displaced by 2-mercaptobenzothiazole ("MBT") to create a crosslink precursor site on the rubber.
When carbon black alone is used as the reenforcing agent in an elastomeric composition, the carbon black surface provides many sites for interacting with the elastomer. While the use of a coupling agent with carbon black might provide some limited improvement in performance to the resulting elastomeric composition, the improvement is not comparable to that obtained when using a silane coupling agent with silica fillers.
It is an object of the present invention to provide novel elastomeric compositions incorporating fillers and treatment agents which can be readily compounded into the elastomer. It is another object, in accordance with certain preferred embodiments, to provide elastomeric compositions incorporating fillers and treating agents exhibiting good hysterisis balance for application in tire tread and other industrial rubber products and other rubber goods. It is yet another object in accordance with certain embodiments to provide a reenforcing agent which includes silicon-treated carbon black and treatment agent. Other objects and features of the present invention will become apparent from the following description and claims.